On 12/07/2017 06:43 PM, Dan Werthimer wrote:
hi xavier,
we use 8 bit wideband digitizers at arecibo and parkes where the RFI
is fairly strong.
do you think you might be able to get away with 8 bits ? :
perhaps you know all this adc and dsp dynamic range stuff already, but
here are some thoughts:
an 8 bit digitizer usually works well if the total power of the RFI
doesn't exceed about 10 times the total power from the noise from the
whole band.
(eg: RFI can be incredibly strong if it's confined to a relatively
narrow band compared to the total band).
although this general rule doesn't apply if the RFI is impulsive (see
below).
have you studied the adc histogram from an 8 bit ADC at your site?
how often does the ADC saturate?
eg: if you set the signal level so the ADC RMS is around 5 (on a scale
from -128 to +127),
how often do you get -128 or +127?
if you have strong impulsive RFI that occasionally saturate the ADC.
there are some casper blocks that can excise these low duty cycle events.
if the total power from the RFI is significantly stronger than the
noise power from the total band,
then you might consider using an 8 bit ADC, but only using the sign
bit from the ADC.
(use the ADC as a one bit digitzer, that outputs only +1 or -1).
you'll loose some SNR with a one bit digitizer (see
thompson/moran/swenson),
and you'll need to apply the van vleck correction to measure strong
signals accurately,
but you won't have a dynamic range problem with a one bit digitizer.
spectrometers and correlators can have 64 bits or more of dynamic
range with 8 bit or 1 bit digitizers.
(the digital part of your design can have lots of dynamic range,
independent of the ADC range).
but if the ADC saturates, and you don't handle the saturation
appropriately,
then you'll get spectral contamination no matter how many bits you use
in the post-adc signal processing).
best wishes,
dan
Last time I was at Arecibo, we were trying to get an HF/low-VHF riometer
working. Below 150Mhz at Arecibo, it is what could charitably be called
"a mess" :) :)
Even with filters in the front-end that restricted the input bandwidth
to 25-45MHz, we had severe issues. Ironically enough, a couple of
years later,
we tried the same instrument at a site only a few dozen km away where
I live (Ottawa, Canada area) and we got excellent results. This was
narrowband stuff, and 8-bits was more than adequate.
On Thu, Dec 7, 2017 at 2:10 PM, Xavier Bosch
<[email protected] <mailto:[email protected]>> wrote:
Hi CASPERites,
We are developing a wideband spectrometer (~1.6 GHz ) in a band
that has a lot of RFI. Currently we are using the 8-bit 5-GSPS
ASIAA ADC board and the ROACH2, and the ADC resolution appears to
be inadequate for our extreme RFI environment.
We would like to move over to a higher resolution ADC (12 to 14
bits), probably with a JESD204B interface, and a Xilinx
development kit. We are considering something like this evaluation
board KCU105
<https://www.xilinx.com/products/boards-and-kits/kcu105.html#hardware>and
something like FMC217
<https://www.vadatech.com/product.php?product=528&catid_prev=0&catid_now=0>
as a digitizer.
Has anyone been in a similar situation dealing with RFI?
Has anyone ported CASPER library to Xilinx development kits and
JESD204B ADCs?
Thank you,
XB
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